Turbogenerators are high advance electrical machines which are connected typically with a gas- or steamturbine arrangement for the purpose of generation of electrical power. The major elements of a turbogenerator are a rotor rotating about an axis and a stator which concentrically surrounds the rotor. Examples of turbogenerators are disclosed in the Article by K. Weigelt, “Konstruktionsmerkmale grosser Turbogeneratoren”, ABB Technik January 1998, pages 3 to 14.
One end section of a typical rotor 1 is illustrated in FIG. 2 which is disclosed in the document DE 10327976 A1 corresponding to UK GB 2403074. The rotor 1 provides at its one end a winding head 2 which connects individual conductors 3 to one another to form a winding of the rotor. The conductors 3 are combined into blocks and inserted into corresponding longitudinal slots 4 of the rotor 1. Whereas the conductors 3 being electrically insulated are secured by wedges 5 in the radial direction, the winding head 2 is secured by means of a retaining ring 6, which is only partly shown in FIG. 2. The retaining ring 6 essentially comprises an annular body and is shrunk onto the rotor 1 radially on the outside in the region of the winding head 2. In this case the retaining ring 6 is designed as a solid steel body in order to be able to absorb the centrifugal forces occurring during operation of the machine equipped with the rotor 1, i.e. an electric motor or generator, in particular a turbogenerator of a power plant.
Provided radially between the retaining ring 6 and the winding head 2 is a damper ring arrangement 7 which is segmented typically, i.e. consists of at least one damper ring having a slit for traversing the damper ring in its circumferential direction, and encloses the winding head 2 in circumferential direction. Furthermore a plurality of different insulating layers 8, 9, 10 and a further plurality of so called damper segments 11 are provided, which are arranged in a radially encircling manner between the retaining ring 6 and the winding head 2. Hereby the damper segments 11, which comprises a multitude of plate-like finger elements disposed in circumferential direction side by side and being connected with each other, electrically contact at one side end with the plurality of wedges 5, which are of electrically conductive material, and otherwise contact the damper ring arrangement 7. Therefore the wedges 5 in combination with the damper segments 11 and the damper ring arrangement 7 form a so called damper cage extending over the whole rotor core and winding assembly. The chief purpose of this damper cage is to improve the electrical damping properties and the operating reliability of the rotor, by allowing a defined compensation current flow which relieves the rotor body electrically and also protects other parts of the rotor. This permits the generator to be operated under special conditions and enables it to cope with certain line faults without damage.
During operation all components of the rotor are subjected to thermal expansion and large centrifugal forces so that especially the wedges 5, the damper segments 11 and the damper rings 7 stay in loose contact to ensure relative movement. The damper segments 11 are in the form of structured plate-like ring bands, each of which is formed to enclose the rotor segmentally, so that in the overall arrangement at least two damper segments 11 enclose the rotor in circumferential direction completely. Each damper segment 11 provides a multitude of plate-like finger elements which are identical in size and shape providing a longitudinal extension and being arranged parallel to each other. The multitudes of plate-like finger elements are arranged side by side so that each neighboring pair of finger elements encloses a gap. All finger elements are joined at one end by a band-like strip section typically in one piece. The band-like strip section provides a longitudinal extension which is oriented orthogonally to the longitudinal extension of the finger elements. Integrated in the rotor, the longitudinal extension of the band-like strip section is oriented in the circumferential direction of the rotor whereas the longitudinal extensions of all plate like finger elements are directed axially to the rotor. Each plate like-finger element stays in loose contact with its radial upper surface with a radial inner surface of an end portion of a wedge, so that mechanical and electrical contact is ensured in operation mode of the electrical machine due to acting centrifugal forces.
Due to different kinds of uncontrolled movement between each plate-like finger element and the surface of a corresponding wedge mutual dislocations may be occur which can lead to altered contact surfaces or even mutual mechanical stresses between the wedges and the finger elements. This however deteriorates the above-mentioned function of the whole damper cage.